Export Skybox for Games
Each UVmap always contains the UV coordinates of all the object's faces.

If the material is the same for each face, I would suggest to make 1 material with 1 big texture that contains all 6 individual textures. Each face's UV island should cover to only a portion of the UV layout as it's done in this related answer https://blender.stackexchange.com/a/28349/15140



This is just one of the possible UV layouts. Sometimes can be useful to have a 3x2 matrix of images and using a rectangular texture... it dependes on your needs.